This incorporates @markshannon's suggestion that the fast locals proxy only access the underlying frame storage, and not support writing additional keys to the dynamic snapshot stored in the C level f_locals struct field.

It also summarises the proposed CPython implementation changes, and cleans up quite a few old references to the idea of storing an instance of the fast locals proxy in the C level f_locals struct field (that hasn't been part of the proposal for ages, but it turned out there were still a lot of references to things working that way that I had missed).